Surface crystallization process

ABSTRACT

Improved purity is achieved in a process for separating the isomers of nitro- and halo-substituted aromatic compounds in a mixture of liquid isomers by fractional, surface crystallization from the melt, wherein the isomeric mixture is cooled until crystals form on a cooling surface. The improvement comprises conducting the fractional, surface crystallization on a glass cooling surface. The process is particularly beneficial for separating para-nitrochlorobenzene from a liquid isomeric liquid of ortho-, meta- and para-nitrochlorobenzenes.

waited States Patent [191 Hug et a1.

[ 1 Aug. 27, 1974 SURFACE CRYSTALLIZATION PROCESS [75] Inventors: Delmar 0. Hug, Edwardsville;

Taniel A. Garabedian, Belleville, both of 111.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: June 7, 1973 [21] Appl. No.: 367,993

Related US. Application Data [63] Continuation-impart of Ser. No. 150,725, June 7,

1971, abandoned.

Robinson et a1 165/133 3,067,270 12/1962 Weedman 260/674 3,272,875 9/1966 Gordon et al 260/646 3,311,666 3/1967 Dunn 260/646 Primary Examiner-Leland A. Sebastian Attorney, Agent, or Firm-Neal E. Willis; John E. Maurer; Frank D. Shearin [57] ABSTRACT Improved purity is achieved in a process for separating the isomers of nitroand halo-substituted aromatic compounds in a mixture of liquid isomers by, fractional, surface crystallization from the melt, wherein the isomeric mixture is cooled until crystals form on a cooling surface. The improvement comprises conducting the fractional, surface crystallization on a glass cooling surface. The process is particularly beneficial for separating para-nitrochlorobenzene from a liquid isomeric liquid of ortho-, metaand paranitrochlorobenzenes.

6 Claims, No Drawings SURFACE CRYSTALLIZATION PROCESS BACKGROUND OF THE INVENTION This application is a continuation-in-part of application Ser. No. 150,725, filed June 7, 1971 and now abandoned.

The present invention relates to crystallization processes, and more specifically relates to separating isomers from a mixture of isomers using crystallization from melt. I

The separation of chemical compounds and chemical isomers by means of crystallization is widely used in American industry. While many separations can be made by distillation or by solvent extraction, there are cases where these methods are impractical, and the desired separation can be achieved more advantageously by means of crystallization. In the case of chemical isomers having physical and chemical properties, or in the case of thermally unstable substances, separation by crystallization may be the only method that can be advantageously employed.

As part of the process for manufacturing organic compounds, such as nitroand halosubstituted aromatic compounds, the problem of separating isomers and homologs-of these compounds in high purity is a major problem. Industry has been interested in obtaining pure forms of these isomers or homologs, since these materials frequently have different chemical or biological properties that make them particularly desirable in many applications. As an example, paranitrochlorobenzene is used in the manufacture of disodium-benzene-meta-disulfonate, para-aminophenol, various pharmaceuticals and a variety of anti-oxidants and gum inhibitors. On the other hand, orthonitrochlorobenzene is used as an intermediate in the synthesis of ortho-aminophenol, dyestuffs, intermediate and photographic developer, chrome-dyestuffs intermediate, and many other intermediate chemicals. It is also used as a solvent in synthetic-petroleum processes. Furthermore, the isomers of nitrochlorobenzene have been very difficult to obtain in high purity because their separation by distillation is extremely difficult due to the closeness of their boiling points, and prior art processes for separating these isomers by crystallization have produced products having a purity of 'only about 95 percent.

Crystallization processes can be carried out either in the presence of a solvent, called crystallization from solution, or in the absence of a solvent, called crystallization from melt. Crystallation from solution separates the components in a mixture by using the differences in solubility at specific temperatures. Crystallization from melt, on the other hand, makes use of the differences in crystallizing points. In the latter case the melt liquor, which is at a temperature where all of the components are soluble in the melt, is gradually cooled until crystals with the highest crystallizing point are formed. Thereafter, the solid crystals with the highest crystallizing point are separated from the melt liquor.

There are two types of fractional crystallization, namely, suspension crystallization and surface crystallization. ln suspension crystallization the crystals are formed in the mother liquor and subsequently separated from the mother liquor such as by filtration or centrifugation. In surface crystallization, the crystals form or nucleate on a cooled surface and the mother liquor is separated from the cooled surface containing the crystals. On a commercial basis surface crystallization is widely used for various separations, such as for the separation of isomers and chemical homologs.

Despite the fact that the art of processes for surface crystallization of isomers is well developed, the prior art fails in one or more ways to teach the advantages of the present invention. As will be seen from the following description, it has been surprisingly found that the process of the present invention provides higher quality crystals with increased purity than can be obtained by means of the prior art processes.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved process for the separation of isomers by surface crystallization. Another object of the present invention is to provide an improved process for the separation of isomers of nitroand halo-substituted aromatic compounds by surface crystallization. Another object of the present invention is to provide an improved process for the separation of paranitrochlorobenzene from a liquid isomeric mixture of ortho-meta-, and para-nitrochlorobenzenes by surface crystallization.

These and other objects are provided in a process for separating the isomers of nitroand halo-substituted aromatic compounds in a liquid isomeric mixture by surface crystallization from, the melt wherein the isomer mixture is cooled until crystals form on a cooling surface, the improvement comprising conducting the surface crystallizationon a cooling surface of glass.

The nitroand halo-substituted aromatic compounds to which this invention pertains include a number of compounds such as nitrochlorobenzene, nit'rofluorobenzene, nitrobromobenzene, nitrochloronaphthalene, nitrobromonaphthalene, dinitrochlorobenzene, dinitrofluorobenzene, dinitrobromobenzene, dinitroiodobenzene, dinitrochloronaphthalene, dinitrofluoronaphthalene, dinitrobromonaphthalene, dinitroiodonaphthalene, dichloronitrobenzene, difluoronitrobenzene, dibromonitrobenzene, diiodonitrobenzene, nitrofluorochlorobenzene, nitrochlorobromobenzene and the like. The process has been found to be particularly effective to separate the isomers of nitrochlorobenzene.

Broadly described, the isomers of nitroand halosubst ituted aromatic compounds are separated by surface crystallization according to the improved process of this invention by introducing an isomeric mixture to be separated into a vessel containing a cooling surface, maintaining the temperature of the cooling surface at such a temperature to deposit the isomer on the cooling surface, separating the cooling surface containing the isomer from the remaining liquid mixture, and then recovering the isomer from the cooling surface. In the present invention, improved isomer purity is achieved by using a cooling surface of glass.

In a typical crystallization process using surface crystallization from melt, the isomer mixture to be separated is introduced to a vessel containing a cooling surface. The vessel need not be of any particular size or shape. The walls of the vessel can act as the cooling surface or alternatively, a cooling surface can be mounted within the vessel. In a typical surface crystallization apparatus the cooled surface which contacts the mother liquor is usually a tube, and traditionally, a metal tube. The tubes thus provide cold surfaces upon which the crystals can form. Conventional tube heat exchangers with a plurality of tubes can be employed for surface crystallization. It has been found to be convenient to use a vessel containing cooling tubes therein for the process of this invention and this is the apparatus we prefer. By way of illustration and not by way of limitation, a specific embodiment of a vessel and coolng tube which can be covered with glass for the process of the present invention, is shown in U.S. Pat. No. 3,219,722, issued Nov. 23, 1965.

Sufficient feed stock is then passed into the vessel to immerse a major proportion of the cooling surface. If the flow of the isomeric mixture is discontinued, it is customary to agitate the liquid in the vessel. However, in some instances the liquid in the vessel is permitted to remain substantially static while crystallization proceeds. Alternatively, a given batch of mixture can be continually circulated into and out of the crystallizer vessel by means of an externalpump, thus causing the liquid to pass over the cooling surface in a continuous manner. It was further discovered that the improved results achieved by the present process are obtained whether the flow of isomeric mixture is parallel to or perpendicular to the surface of the cooling surface. In addition, it was found that the purity of the desired isomer obtained by the present process was achieved whether the isomeric mixture was agitated mechanically within the vessel, or allowed to remain static within the vessel, or was circulated through the vessel by an external, pump.

The temperature of the cooled surface is maintained at the temperature necessary to deposit the isomer. Any number of means well known to those skilled in the art can be used to maintain the necessary temperature. As an example, if the walls of the vessel are used as the cooling surface, the temperature of the walls can be controlled by various heaters, various c oolants or a combination of heating means and cooling means. In a typical surface crystallization apparatus the cooled surface which contacts the isomeric mixture usually comprises cooling conduits in the form of tubes in which the cold surfaces upon which the isomer can deposit are cooled by passing a coolant through the cooling conduits. The coolant can be any fluid medium that will provide the necessary cooling of the feed stock or the mother liquor. Representative coolants are precooled fluids such as water, brine, mother liquor, methanol and the like; evaporative coolants, such as fluorinated hydrocarbons, propane, ammonia and the like, or gases, such as air, nitrogen or the like.

The cooling surfaces can be covered with glass by any number of means known to those skilled in the art. Where the term glass is used herein, it is understood to mean a smooth form of those materials which are fused mixtures of the silicates of the alkali metals, alkaline earth metals or heavy metals. The glass can be applied to the surface as a powder or spray. Then, the surface can be heated in a furnace to fuse the glass particles and convert them to an impervious coating. The cooling surface can also be coated with glass by spraying molten glass against the surface using a plasma arc spray or the like. On the other hand, when the cooling surface is a metal tube which is removable from the crystallizer vessel in straight cylinderical sections, a close-fitting, glass tube can be conveniently slipped over the metal tube, and the double-tube assembly can then be reinstalled in the crystallizer vessel. Where the metal tube configuration does not permit the installation of an integral glass tube converging, a split-tube design can be employed. Thus, two longitudinal halves of the glass tube of appropriate length can be applied to the metal tube and bonded thereto.

Considerable range is permitted in the thickness of the glass tubing or glass coating which can be employed in the process of this invention. Conveniently, the glass thickness can range from about 0.5 to about 5 times of the thickness of the conventional metal tubes. Since thermal conductivity is a function of material thickness, the glass thickness may be dictated by the characteristics of the coolant supply system, e.g., the nature of the coolant, the rate of coolant flow and the coolant temperature.

After the isomer has deposited on the cooling surface, the cooling surface containing the isomer is separated from the remaining liquid mixture by any number of means known to the art. If the cooling surface is removably mounted in the crystallizer vessel, the cooling surface containing the deposited isomer can be removed mechanically from the crystallizer vessel and the remaining liquid mixture. On the other hand, if the cooling surface is permanently mounted on .the crystallizer vessel, or is difficult to remove from the crystallizer vessel, which is the case in a typical crystallizer apparatus, the remaining liquid mixture is drained away from the cooling surface containing the deposited isomer.

After the cooling surface containing the deposite isomer is separated from the remaining liquid mixture, the deposited isomer can be recovered from the cooling surface by any number of means known to the art. As an example, the isomer can be scraped from the cooling surface mechanically. On the other hand, the cooling surface can be heated to a temperature above the melting point of the isomer to melt the isomer off the cooled surface and this is the procedure we prefer to use. In a typical crystallization process a heated fluid is passed through the cooling conduits, after the remaining liquid mixture is drained away from the cooling surface, to melt the deposited isomer which is then collected as the liquid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS sel, and water flowing through the cooling tubes. Conventional steel tubes were used as the cooling surface except that one of the steel tubes was covered with a cylindrical glass sleeve having a thickness of approximately 2 milliliters. The temperature of the cooling water was gradually reduced during each test such that the temperature of the crude nitrochlorobenzene mixture was also reduced to achieve deposition of the para isomer on the cooling surface. The volume flow of the crude mixture through the recirculating loop could be varied from 5 to gallons per minute. The preferred velocity of the crude mixture over the cooling tubes was about 0.08 feet per minute.

After the batch of crude mixture had circulated through the crystallizer for several hours, the crystal cake on a plain steel cooling tube was analyzed and compared to the analysis of the crystal cake on the glass-covered steel cooling tube. In each case the para isomer of nitrochlorobenzene was present in greater amount in the crystal cake removed from the glass covered cooling tube. Furthermore, the usual sweating or dripping effect, characteristically present on the metal tubes, was not found on the glass-covered tube. In addition, the crystals of para-nitrochlorobenzene deposited on the glass-covered tube were icelike in appea'rance, thus being physically distinguishable from the opaque crystals deposited on the conventional steel tube.

This invention is further illustrated by but not limited to the following examples which show the results of the above comparative tests.

EXAMPLE 1 About 38,650 grams of a crude nitrochlorobenzene mixture (89.85 percent para, 9.36 percent ortho, and 0.78 percent meta) were circulated through the crystallizer as described above for approximately 2 hours while the temperature of the crude mixture was slowly reduced from 85 to 78 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 91.72 percent para, 7.57 percent ortho and 0.71 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 94.81 percent para, 4.94 percent ortho and 0.25 percent meta.

EXAMPLE 2 About 38,650 grams of a crude nitrochlorobenzene mixture (90.00 percent para, 9.22 percent ortho and 0.78 percent meta) were circulated through the crystallizer for approximately 4 1/2 hours while the temperature of the crude mixture was slowly reduced from 855 to 765 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 96.20 percent para, 0.56 percent ortho and 0.23 percent meta. The crystals removed from the glasscovered cooling tube were analyzed as 99.72 percent para, 0.23 percent ortho and 0 percent meta.

EXAMPLE 3 About 38,650 grams of a crude nitrochlorobenzene mixture (90.00 percent para, 9.22 percent ortho and 0.78 percent meta) were circulated through the crystallizer for approximately 2 hours while the temperature of the crude mixture was slowly reduced from 85 to 78 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 93.69 percent para, 6.00 percent ortho and 0.31 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 98.95 percent para, 0.94 percent ortho and 0.1 1 percent meta.

EXAMPLE 4 About 38,650 grams of a crude nitrochlorobenzene mixture (90.11 percent para, 910 percent ortho and 0.79 percent meta) were circulated through the crystallizer for approximately 5% hours while the temperature of the crude mixture was slowly reduced from 85 to 75 C. The nitrochlorobenzene crystals removed from a plain steel cooling ube were analyzed as 97.83 percent para, 2.07 percent ortho and 0.10 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 99.63 percent para, 0.34 percent ortho and 0.03 percent meta.

EXAMPLE 5 EXAMPLE 6 About 33,000 grams of a crude nitrochlorobenzene mixture (90.19 percent para, 9.04 percent ortho and 0.77 percent meta) were circulated through the crystallizer for approximately 2%; hours while the temperature of the crude mixture was slowly reduced from C. to 74 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 94.86'percent para, 4.93 percent ortho and 0.21 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 97.17 percent para, 2.70 percent ortho and 0.13 percent meta.

EXAMPLE 7 About 33,000 grams of a crude nitrochlorobenzene mixture (90.14 percent para, 907 percent ortho and 0.77 percent 'meta) were circulated through the crystallizer for approximately 3 /3 hours while the temperature of the crude. mixture was slowly reduced from 81.4 C. to 790 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 96.64 percent para, 3.22 percent ortho and 0.09 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 99.45 percent para, 0.48 percent ortho and 0.07 percent meta.

EXAMPLE 8 About 33,000 grams of a crude nitrochlorobenzene mixture (90.25 percent para, 8.94 percent ortho and 0.81 percent meta) were circulated through the crystallizer for approximately 3% hours while the temperature of the crude mixture was slowly reduced from 81.7 C. to 780 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 96.97 percent para, 2.93 percent ortho and 0.10 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 99.05 percent para, 0.91 percent ortho and 0.04 percent meta.

EXAMPLE 9 About 33,000 grams of a crude nitrochlorobenzene mixture (90.21 percent para, 9.00 percent ortho and 0.79 percent meta) were circulated through the crystallizer for approximately 3% hours while the temperature of the crude mixture was slowly reduced from 8 1 .5 C. to 770 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 97.20 percent para, 2.55 percent ortho and 0.25 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 98.50 percent para, 1.44 percent ortho and 0.06 percent meta.

EXAMPLE 10 EXAMPLE 1 l About 33,000 grams of a crude nitrochlorobenzene mixture (90.28 percent para, 8.91 percent ortho and 0.81 percent meta) were circulated through the crystallizer for approximately 6 hours while the temperature of the crude mixture was slowly reduced from 81 .5 C. to 740 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 97.65 percent para, 2.27 percent ortho and 0.08 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 99.19 percent para, 0.76 percent ortho and 0.05 percent meta.

EXAMPLE 12 About 33,000 grams of a crude nitrochlorobenzene mixture (90.32 percent para, 8.81 percent ortho and 0.81 percent meta) were circulated through the crystallizer for approximately 6 /2 hours while the temperature of the crude mixture was slowly reduced from 80.8 C. to 74.l C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 97.67 percent para, 2.23 percent ortho and 0.10 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 98.28 percent para, 1.63 percent ortho and 0.09 percent meta.

EXAMPLE 13 About 33,000 grams of a crude nitrochlorobenzene mixture (91.15 percent para, 8.08 percent ortho and 0.77 percent meta) were circulated through the crystallizer for approximately 6 hours while the temperature of the crude mixture was slowly reduced from 818 C. to 730 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 96.48 percent para, 2.08 percent ortho and 0.14 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 98.78 percent para, 1.16 percent ortho and 0.06 percent meta.

EXAMPLE 14 About 38,650 grams of a crude nitrochlorobenzene mixture (89.92 percent para, 9.29 percent ortho and 0.79 percent meta) were circulated through the crystallizer fro approximately 4 hours while the temperature of the crude mixture was slowly reduced from C. to 76 C. The nitrochlorobenzene crystals removed from a plain steel cooling tube were analyzed as 94.44 percent para, 5.07 percent ortho and 0.49 percent meta. The crystals removed from the glass-covered cooling tube were analyzed as 96.28 percent para, 3.45 percent ortho and 0.27 percent meta.

It is to be understood that the present invention is not confined to the preferred combination of a cooling surface (e.g., a metallic tube) and a glass sleeve or glass coating. It has also been discovered that a glass tube, per se, is a cooling surface superior to a steel tube, per se. Thus, it was demonstrated that improved purity can be achieved in the surface crystallization of pnitrochlorobenzene when using a plain glass cooling tube, as compared to the purity achieved with a plain steel cooling tube. While it is recognized that plain glass cooling tubes may be impractical in many crystallizer applications for structural reasons, such tubes are within the scope of the present invention.

Although the invention has been described in terms of specified embodiments which are set forth in considerable detail, it should be understood that this is by way of illustration only, and that the invention is not necessarily limited thereto since alternative embodiments and operating techniques will become apparent to those skilled in the art in view of the disclosure. As an example, the preferred embodiments teach that a batch crystallization process was used whereas it is clear that the process as contemplated herein can be applied to a continuous crystallization process. Accordingly, modification are contemplated which can be made without departing from the spirit of the described invention.

What is claimed is:

1. In a process for separating the isomers of nitroand halo-substituted aromatic compounds in a liquid isomeric mixture by surface crystallization from the melt, whereinthe isomeric mixture is cooled until crystals form on a cooling surface, the improvement comprising conducting the surface crystallization on a cooling surface of glass.

2. In a process of claim 1 wherein the cooling surface is a metallic conduit covered with glass.

3. In a process of claim 2 wherein the metallic conduit is steel tubing.

4. In a process of claim 2 wherein the cooling surface is a metal conduit covered with a close fitting glass sleeve on the periphery of the conduit.

5. In a process of claim 2 wherein the cooling surface is a glass sleeve bonded to the periphery of the metallic conduit.

6. In a process of claim 1 for separating paranitrochlorobenzene from a liquid isomeric mixture of ortho-, meta-, and para-nitrochlorobenzene, the improvement comprising conducting the surface crystallization on a cooling surface of glass. 

2. In a process of claim 1 wherein the cooling surface is a metallic conduit covered with glass.
 3. In a process of claim 2 wherein the metallic conduit is steel tubing.
 4. In a process of claim 2 wherein the cooling surface is a metal conduit covered with a close fitting glass sleeve on the periphery of the conduit.
 5. In a process of claim 2 wherein the cooling surface is a glass sleeve bonded to the periphery of the metallic conduit.
 6. In a process of claim 1 for separating para-nitrochlorobenzene from a liquid isomeric mixture of ortho-, meta-, and para-nitrochlorobenzene, the improvement comprising conducting the surface crystallization on a cooling surface of glass. 